In a bowl, mix together the soy sauce, oyster sauce, and cornstarch until smooth. Add the sliced beef, ensuring each piece is well-coated in the marinade. Allow to marinate for at least 15 minutes.

Heat vegetable oil in a large skillet over high heat. Add the marinated beef slices in a single layer, searing for 1-2 minutes on each side until browned. Remove from skillet and set aside.

In the same skillet, add remaining oil. Lower heat to medium and toss in sliced bell peppers and onions. Sauté for 3-4 minutes until tender yet crisp.

Stir in minced garlic and grated ginger, cooking for an additional minute.

Return the seared beef to the skillet, pour in beef broth or water, and stir to combine. Allow to simmer for 2-3 minutes until sauce thickens.

Season with salt and pepper to taste, then serve hot over rice or noodles.